Low frequency induction plasma system

ABSTRACT

A low frequency induction plasma system includes a plasma chamber and energizing coil dimensioned as a function of frequency. Structure for lowering the reluctance of the magnetic path surrounds the coil and the chamber so as to enhance the flux in the chamber produced by the energized coil. A thermal plasma in a flowing system is established by initiating a glow discharge with an auxiliary high frequency power supply and then creating a thermal plasma assisted by the enhanced flux in the chamber.

United States Patent [1 1 Dundas et a1.

LOW FREQUENCY INDUCTION PLASMA SYSTEM Inventors: Peter H. Dundas,Andover, Mass;

John W. Poole; Charles E. Vogel, both of Bow, N.l-l.

Assignee: Humphreys Corporation, Bow, N.l-l.

Filed: Jan. 26, 1973 Appl. No; 327,082

Related US. Application Data Continuation of Ser. No. 173,500, Aug. 20,1971,

abandoned.

us. c1. 219/121 1 1111.01 823k 9/00 Field of Search 219/121 R, 121 P,383,

References Cited UNITED STATES PATENTS 12/1959 Giannini et a1. 219/121 RX 11/1964 Gross 219/121 PX Jan. 21, 1975 3,264,508 8/1966 Lai et al.219/121 P X 3,296,410 1/1967 Hedger 219/121 P 3,543,084 11/1970Michaelis... 219/121 R X 3,571,772 3/1971 Bennon 336/84 3,620,00811/1971 Newbold 219/121 P X 3,639,872 2/1972 Hessen et a1... 336/843,646,308 2/1972 Splessens 219/121 P Primary Examiner.l. V. TruheAssistant Examiner--Ga1e R. Peterson 1 ABSTRACT A low frequencyinduction plasma system includes a plasma chamber and energizing coildimensioned as a function of frequency. Structure for lowering thereluctance of the magnetic path surrounds the coil and the chamber so asto enhance the flux in the chamber produced by the energized coil. Athermal plasma in a flowing system is established by initiating a glowdischarge with an auxiliary high frequency power supply and thencreating a thermal plasma assisted by the enhanced flux in the chamber.

12 Claims, 3 Drawing Figures Patentd Jan. 2-1, 1975 r r 3,862,393

2 Sheets-Sheet 1 LOW FREQUENCY INDUCTION PLASMA SYSTEM This applicationis a continuation of Ser. No. 173,500, filed 'Aug. 20, 1971, nowabandoned, entitled Low Frequency Induction Plasma System.

. The invention described herein was made in the performance of workunder a NASA contract and is subject to the provisions of section 305of, the National Aeronautics and Space Act of' 1958, Public Law 85568(72 Stat. 435; 42 U.S.C. 2457).

SUMMARY OF INVENTION This invention relates to induction plasma systemsand to methods of operating such systems.

A thermal plasma is created and maintained in a system of the inductiontype by providing an intense electromagnetic field which produces anelectrodeless discharge (thermal plasma) inan ionized medium. Such athermal plasma has a temperature in the range of 8,0001 1,000K and isuseful for a variety of purposes including the performance of chemicalreactions, the working ofmetallic and/or refractory materials, heatersfor hypersonic tunnels and simulator devices that utilize the hightemperatures produced by such systems. A number of arrangements havebeen proposed for generating such thermal plasmas, including both closedand flowing systems in which the plasma has been stably maintained bothin reduced pressure environments and in atmospheric pressureenvironments. In general, such devices have the electromagneticfieldproduced by power sources operating in the megahertz and near megahertzfrequency range, and as such require expensive electronic circuitry ofrelatively low efficiency.

It is an'object of this invention to provide a novel and improvedinduction plasma system operable at frequencies well below the megahertzrange.

Another object of the invention is to provide novel and improvedinduction plasma systems of the flowing type. A flowing type of systemfacilitates the performance of useful work, either by the plasma withinthe plasma chamber or by the effluent from the plasma chamber. In suchflowing systems, the stabilization of the plasma, both spacially andelectrically, becomes more critical, as that plasma must be stabilizedunder the dynamic flow conditions against drift which would damage theintegrity of the apparatus and also to maintain the necessary electricalcharacteristics of the system so that the thermal plasma is notextinguished. The extinction of the thermal plasma is a function of theenergizing frequency and the characteristic decay time of the plasma.

According to theory, the maintenance of an induction plasma is dependenton a circulating current which should circulate near the wall of theplasma chamber.

' The depth of penetration of such current may be termed skin depth" andis represented by the formula:

8 skin depth 1- conductivity of load ,1 magnetic permeability of load f=frequency of applied energy For coupling efficiency the skin depthshould be less than the radius of the plasma and therefore the plasmadiameter should be larger at lower frequencies of energization. Aminimum voltage gradient around the plasma is required to sustain theplasma, this gradient being 2 volts/cm in a high current argon arc, andhigher for air. Thus the requisite voltage increases with largerdiameter plasmas. These additional considerations have leadexperimenters in induction plasma systems to employ high frequencyenergizing sources. Through our invention involving particular ignitionand magnetic enhancement techniques, we have successfully operatedstable flowing induction plasma systems at frequencies far below thefrequencies conventionally employed.

In accordance with our invention, there is provided a plasma generationsystem that includes an elongated plasma defining chamber having exhaustport structure at one end and injector structure at the one end forintroducing material plasma forming material at one end of the chamberfor flow through the chamber and exit through the exhaust portstructure. An electrical coil surrounds the chamber and is connected toa relatively low frequency power supply (that is 20 kilohertz or below)for creating an intense electromagnetic field within the chamber.Magnetic enhancement structure that at least doubles the flux densityinthe chamber surrounds the electrical coil. In a particular embodimentthis magnetic enhancement structure is formed of a series of discrete,spaced groups of clamped, axially extending electrical steellaminations, with cooling structures on the sides of the groups adjacentthe leads of the electrical coil. I

Plasma initiation in this low frequency system is preferablyaccomplished by initially reducing the pressure in the chamber andapplying a high frequency (e.g., 20 megahertz) signal across the chamberto establish a glow discharge that substantially fills the chamber andthen coupling the low frequency power supply to the electrical coil toconvert the glow discharge to a thermal plasma. In a particularembodiment this glow discharge condition is established by shortduration pulsed application of high frequency energy.

The methods and apparatus in accordance with the invention provide asimple, reliable, and relatively easy to operate induction plasma systemof the flowing type operable to provide a stable thermal plasmaenergized at low frequencies of less than 20 kilohertz in a system.

Other objects, features and advantages of the invention will be seen asthe following description of a particular embodiment progresses inconjunction with the drawings, in which:

FIG. 1 is a diagrammatic view of an induction plasma system constructedin accordance with the invention;

FIG. 2 is a sectional view showing details of the induction plasmaapparatus shown in FIG. 1 taken along the line 2-2 of FIG. 1; and

FIG. 3 is a diagram of an induction plasma system employing theapparatus as shown in FIG. 1.

DESCRIPTION OF PARTICULAR EMBODIMENT The induction plasma generatorapparatus shown in FIG. 1 is designed for operation at 9600I-Iz andincludes an opaque fused quartz tube I0 having an inner diameter of fiveand one-half inches and a length of twenty and one-half inches. In asystem designed for operation at 960I-Iz a tube 10 that has a diameterof twelve inches and a length of sixty inches is employed. The upstreamend 12 of tube 10 is seated on flange 14 of end plate structure 16 andsealed by O-ring l8, and the downstream end 20 of tube 10 is similarlyseated within recess 22 of end plate 24 and sealed by O-ring 26.

lnjector structure 30 and flow stabilizer structure 32 are positionedinend plate 16 and clamped plate 34 and bolts 36. A first inlet port 38supplies gas through channel 40 to annular chamber 42 for distributionthrough radially directed ports 44. A second inlet port 46 supplies gasthrough passage 48 to annular chamber 50 for distribution through swirlports 52. Axial ports '54 are supplied through a third passage (notshown) that is angularly offset from passages 40 and 48. The upper endof injector structure 30 is cooled by water flowing through passages 56and chamber 58 which flow stabilizer structure 32 is cooled by waterflowing through passages 60 and chamber 62.

A cast acrylic cylinder 64 surrounds tube with its upstream end seatedin recess 66 in end plate 16 and its downstream end seated in recess 68in end plate 24. Pressurized coolant is supplied through passage 70 forflow throughspace 72 and exhaust through passage 74. An induction coil80 is formed of rectangular (one inch by one-half inch) one sixteenthinch wall copper tubing in a coil having an inner diameter of seven andonequarter inches and a length of seven inches. Surrounding coil 80 is amagnetic enhancement structure 82 in the form of a series of sixelectrical steel lamination assemblies 84, as indicated in FIG. 2. Eachsuch assembly includes two clamp plates 86 between which are disposed astack of Armco Tran-cor T electricalsteel laminations 88, eachlamination being 0.007 inch thick, seven-eighths inch wide and ten andone-half inches long, with inwardly extending ear portions 89 at eitherend. Each stack has a depth of two and one-half inches. Each laminationassembly 84 is mounted on support 90. Cooling tubes 92 are secured tothe clamp plates 86 on either side of coil terminals 94 by silverbrazing and cooling water is flowed through tubes 94 during operation ofthe apparatus. This magnetic enhancement structure more than trebles theflux density in chamber End plate 24 has an evacuation port 100 which isarranged for connection via valve 102 to a vacuum systern and an exitport 104 across which is disposed a grid 106 of cooling pipes 107supported on ring 108. Glass plate 110 functions as a valve and as asight port when the system is operating under reduced pressureconditions isseated on ring 108 and is sealed by O-rings 112.

A diagram of the operating system is shown in FIG. 3. The injectorassembly 30 is connected to a suitable source 120 of gas. Top flange 24is connected to vacuum system 122 as controlled by valve 102. Exit port104 is controlled by valve 110. A 450 kilowatt 9600 Hertz motorgenerator power supply 124 and 9200 KVAR capacity is connected to coil80 and high frequency MHz) auxiliary ignition supply 126 connectedthrough switch 128 across end plates 16 and 24.

To form a thermal plasma 130 in chamber 10, valve 110 is closed and thepressure in the chamber 10 is reduced to a pressure of one Torr.,. theRF power supply ignition source 126 is energized and a glow discharge isproduced whichfills chamber 10. With the main (LF) power supply 124energized, the glow discharge provides a sufficient load for coupling bythe low frequency power and a thermal plasma 130 is established assignified by a dramatic change in brightness. Before establishment ofthe plasma 130 the power supply 124 was operating at full voltage, 60percent current and 6 percent lead power factor. After establishment ofplasma, voltage dropped to 40-50 percent current increased to percentand the power factor shifted to 10 percent lag. With initiation of thethermal plasma, argon gas flow is increased, increasing the pressure inthe chamber 10. Plasma is stably maintained as the pressure is increasedto atmospheric pressure in a flowing mode of operation. With an argonflow rate of approximately 700 SCFH at one atmosphere pressure, theminimum sustaining power was KW.

In a 960 Hz system, a 1250 KW motor generator power supply 124' wasemployed with a capacitor bank of 22,000 KVAR capacity connected to coil80'. The high frequency ignition supply was a 25 KW 4 MHZ supply. Glowdischarge at 0.35 Torr was produced by energizing coil 80 and thenapplying a pulse (172 millisecond in duration in one sequence) ofignition energy across end plates 16', 24' to create a glow dischargethat fills chamber 10' and the low frequency enhanced electromagneticfield couples to the discharge and a plasma 130 is established.

While a particular embodiment of the invention has been shown anddescribed, various modifications thereof will be apparent to thoseskilled in the art and therefore it is not intended that the inventionbe limited to the disclosed embodiment or to details thereof anddepartures may be made therefrom within the spirit and scope of theinvention as defined in the claims.

- What is claimed is:

1. An induction plasma system for use with an ignition sourcecomprising:

structure defining an elongated plasma chamber,

an electrical coil surrounding said chamber,

' means for supplying a plasma forming material for flow into saidchamber and conversion to plasma condition under the influence of anelectromagnetic field produced by said electrical coil, and exhaust portstructure at one end of said chamber defining a port for flow of saidmaterial from said chamber, wherein the improvement comprises a lowfrequency power supply connected to said coil for creating an intenseelectromagnetic field within said chamber, and

magnetic enhancement structure disposed adjacent said coil, saidmagnetic enhancement structure providing magnetic flux density in saidchamber that is at least twice the flux density that exists in saidchamber in the absence of said magnetic enhancement structure.

2. The system as claimed in claim 1 wherein said magnetic enhancementstructure comprises a series of spaced groups of axially extendinglaminations of electrical steel that surround said coil.

3. The system as claimed in claim 2 and further including cooling meansdisposed between the leads of said coil and each immediately adjacentgroup of laminations.

4. The system as claimed in claim 1 and further including an auxiliaryhigh frequency power supply and means to connect said auxiliary powersupply across the ends of said plasma chamber and establish a glowdischarge in said plasma chamber.

5. The system as claimed in claim 1 and further including means forsealing said exhaust port structure and further including auxiliaryexhaust port structure of substantially smaller dimension than thedimension of saidmain exhaust port structure.

6. The system as claimed in claim 5 wherein said plasma chamber definingstructure is a tubular member of uniform cross-sectional configurationthroughout its eluding means to'reduce the pressure in said plasmachamber, an auxiliary high frequency power supply and means to connectsaid auxiliary power supply across the ends of said plasma chamber andestablish a glow discharge in a reduced pressure environment is saidplasma chamber.

9. A method for initiating operation of a low frequency inductionflowing plasma system having a plasma defining chamber having a gas'inlet port at one end and a plasma outlet port at the opposite end, andan' electrical coil surrounding said chamber and adapted to be connectedto a suitable low frequency power supply for creating an intenseelectromagnetic field within said chamber comprising the steps ofcontinuously applying said low frequency energy to said coil, reducingthe pressure in said chamber,

establishing a glow discharge in said chamber by applying a dischargepulse of high frequency energy across the ends of said plasma chamber tocreate a conductive condition in the reduced pressure environment insaid chamber, said conductive condition enabling a thermal plasma to beestablished in said chamber under the influence of the electromagneticfield created by the low frequency energy applied to said electricalcoil, and after said thermal plasma is established, increasing thepressure in said chamber to atmospheric pressure while continuouslyflowing gas through said inlet port into said chamber so that plasmaflows through said outlet port.

10. The system as claimed in claim 1 wherein said power supply has anoperating frequency of less than 20 kilohertz for creating anelectromagnetic field within said plasma chamber inside said electricalcoil.

11. An induction plasma system for use with an ignition sourcecomprising:

structure defining an elongated plasma chamber,

an electrical coil surrounding and extending along an axial length ofsaid plasma chamber,

inlet port structure for supplying a plasma forming material for flowinto said chamber and conversion to plasma condition under the influenceof an electromagnetic field produced by said electrical coil,

, and exhaust port structure at one end of said chamber defining a portfor flow of said material in plasma condition from said chamber, whereinthe improvement comprises a low frequency power supply connected to saidcoil for creating an intense electromagnetic field within said chamber,and

magnetic enhancement structure disposed adjacent I and outside of andextending around said coil and said chamber, said magnetic enhancementstructure including a main portion extending the length of said coil andportions at either end of said main portion that overlie the-ends ofsaid coil and extend inwardly from said main portion towards said plasmachamber.

12. The system as claimed in claim 11 wherein said magnetic enhancementstructure comprises a series of spaced axially extending members ofmagnetic material, said series of members surrounding said coil and eachsaid member including an inwardly extending end portion that overliesthe adjacent end of said coil, said magnetic enhancement structure beingisolated from the high voltage that appears across the ends of saidcoil.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,62,393

DATED 1 January 21, 1975 INVENTOR(S) Peter H. Dundas et a1.

it is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 6, lines 5-6, delete "for creating, an

electromagnetic field within said nlasma. chamber inside said electricalcoil".

Signed and sealed this 6th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks

1. An induction plasma system for use with an ignition source comprising: structure defining an elongated plasma chamber, an electrical coil surrounding said chamber, means for supplying a plasma forming material for flow into said chamber and conversion to plasma condition under the influence of an electromagnetic field produced by said electrical coil, and exhaust port structure at one end of said chamber defining a port for flow of said material from said chamber, wherein the improvement comprises a low frequency power supply connected to said coil for creating an intense electromagnetic field within said chamber, and magnetic enhancement structure disposed adjacent said coil, said magnetic enhancement structure providing magnetic flux density in said chamber that is at least twice the flux density that exists in said chamber in the absence of said magnetic enhancement structure.
 2. The system as claimed in claim 1 wherein said magnetic enhancement structure comprises a series of spaced groups of axially extending laminations of electrical steel that surround said coil.
 3. The system as claimed in claim 2 and further including cooling means disposed between the leads of said coil and each immediately adjacent group of laminations.
 4. The system as claimed in claim 1 and further including an auxiliary high frequency power supply and means to connect said auxiliary power supply across the ends of said plasma chamber and establish a glow discharge in said plasma chamber.
 5. The system as claimed in claim 1 and further including means for sealing said exhaust port structure and further including auxiliary exhaust port structure of substantially smaller dimension than the dimension of said main exhaust port structure.
 6. The system as claimed in claim 5 wherein said plasma chamber defining structure is a tubular member of uniform cross-sectional configuration throughout its axial length and said exhaust port sealing means includes a removable plate member disposed across the downstream end of said plasma chamber defining structure.
 7. Ths system as claimed in claim 5 wherein said magnetic enhancement structure comprises a series of spaced groups of axially extending laminations of electrical steel that surround said coil and further including cooling means disposed between the lead of said coil and each immediately adjacent group of laminations.
 8. The system as claimed in claim 7 and further including means to reduce the pressure in said plasma chamber, an auxiliary high frequency power supply and means to connect said auxiliary power supply across the ends of said plasma chamber and establish a glow discharge in a reduced pressure environment is said plasma chamber.
 9. A method for initiating operation of a low frequency induction flowing plasma system having a plasma defining chamber having a gas inlet port at one end and a plasma outlet port at the opposite end, and an electrical coil surrounding said chamber and adapted to be connected to a suitable low frequency power supply for creating an intense electromagnetic field within said chamber comprising the steps of continuously applying said low frequency energy to said coil, reducing the pressure in said chamber, establishing a glow discharge in said chamber by applying a discharge pulse of high frequency energy across the ends of said plasma chamber to create a conductive condition in the reduced pressure environment in said chamber, said conductive condition enabling a thermal plasma to be established in said chamber under the influence of the electromagnetic field created by the low frequency energy applied to said electrical coil, and after said thermal plasma is established, increasing the pressure in said chamber to atmospheric pressure while continuously flowing gas through said inlet port into said chamber so that plasma flows through said outlet port.
 10. The system as claimed in claim 1 wherein said power supply has an operating frequency of less than 20 kilohertz for creating an electromagnetic field within said plasma chamber inside said electrical coil.
 11. An induction plasma system for use with an ignition source comprising: structure defining an elongated plasma chamber, an electrical coil surrounding and extending along an axial length of said plasma chamber, inlet port structure for supplying a plasma forming material for flow into said chamber and conversion to plasma condition under the influence of an electromagnetic field produced by said electrical coil, and exhaust port structure at one end of said chamber defining a port for flow of said material in plasma condition from said chamber, wherein the improvement comprises a low frequency power supply connected to said coil for creating an intense electromagnetic field within said chamber, and magnetic enhancement structure disposed adjacent and outside of and extending around said coil and said chamber, said magnetic enhancement structure including a main portion extending the length of said coil and portions at either end of said main portion that overlie the ends of said coil and extend inwardly from said main portion towards said plasma chamber.
 12. The system as claimed in claim 11 wherein said magnetic enhancement structure comprises a series of spaced axially extending members of magnetic material, said series of members surrounding said coil and each said member including an inwardly extending end portion that overlies the adjacent end of said coil, said magnetic enhancement structure being isolated from the high voltage that appears across the ends of said coil. 